1. Technical Field
The disclosure relates to a steering apparatus.
2. Description of Related Art
Conventionally, there is a steering apparatus for a vehicle (an automobile), the steering apparatus being configured to assist an operation of a rack shaft by generating an axial thrust of the rack shaft by an electric motor (see Japanese Patent Application Publication No. 2014-77459 (JP 2014-77459 A)). In the steering apparatus described in JP 2014-77459 A, a rolling element screw portion (a ball screw portion) is formed on an outer peripheral surface of the rack shaft. A rolling element nut (a ball nut) is threadedly engaged with the rolling element screw portion via a plurality of rolling elements (balls) having the same diameter so as to be engaged with a groove of the rolling element screw portion, thereby constituting a ball screw mechanism. In the ball screw mechanism, the rolling element nut is rotated around the rack shaft by a rotational force of the electric motor. Accordingly, the rolling element nut thus rotated moves the rack shaft in an axis direction via the plurality of rolling elements. The steering apparatus described in JP 2014-77459 A is a so-called rack-parallel steering apparatus configured to assist an operation of the rack shaft by generating an axial thrust for the rack shaft with the use of an electric motor configured to be separated from the rack shaft and having an output shaft provided in parallel with an axis of the rack shaft. In the steering apparatus described in JP 2014-77459 A, an external thread is formed on the outer peripheral surface of the rack shaft. An internal thread formed on an inner peripheral surface of the rolling element nut (the ball nut) is threadedly engaged with the external thread of the rack shaft via the plurality of rolling elements (balls) having the same diameter, thereby constituting the ball screw mechanism. The rolling element nut is formed in a cylindrical shape, and one end thereof is supported by a housing of the steering apparatus. Further, the other end of the rolling element nut is connected to the output shaft of the motor via a toothed belt, so as to be rotationally driven by the motor. Thus, the rolling element nut is rotated by the motor so as to move the rack shaft in the axis direction via the plurality of rolling elements.
Note that, in the ball screw mechanism described in JP 2014-77459 A, the diameter and the number of the plurality of rolling elements arranged and accommodated in the groove of the rolling element screw portion are set such that the rolling elements can be arranged with a prescribed interval between the rolling elements adjacent to each other in the groove. Therefore, when the rolling element nut rotates around the axis relative to the rolling element screw portion with which the rolling element is threadedly engaged, each of the rolling elements contacting a groove surface of a groove of the rolling element nut and a groove surface of a groove of the rolling element screw portion is rolled on the surfaces of the grooves at a constant speed in the same direction without contacting its adjacent rolling elements. Thus, it is possible to smoothly rotate the rolling element nut and the rolling element screw portion relative to each other with a low resistance.
Further, conventionally, there is a steering apparatus for a vehicle (an automobile), the steering apparatus being configured to assist an operation of a rack shaft by generating an axial thrust for the rack shaft with the use of an electric motor (see Japanese Patent Application Publication No. 2014-105802 (JP 2014-105802 A)). In the steering apparatus described in JP 2014-105802 A, a rolling element external thread portion is formed on an outer peripheral surface of the rack shaft. Further, a rolling element internal thread portion is formed on an inner peripheral surface of a rolling element nut. The rolling element external thread portion and the rolling element internal thread portion form a rolling element screw portion, and are threadedly engaged with each other via a plurality of rolling elements (balls) having the same diameter. Further, the rolling element internal thread portion is formed such that an internal thread diameter of its axial central part is smaller than an internal thread diameter of its axial end. It is described that this configuration prevents a decrease in durability due to an excessive load being applied to the rolling elements (balls) disposed in the axial end of the rolling element screw portion from the rack shaft that is bent when a large load is input into the rack shaft to cause a prescribed amount of bending in the rack shaft.
Note that, in a ball screw mechanism described in JP 2014-105802 A, the diameter and the number of the plurality of rolling elements arranged and accommodated in the groove of the rolling element screw portion are set such that the rolling elements can be arranged with a prescribed interval between the rolling elements adjacent to each other in the groove. Therefore, when the rolling element nut is rotated around the axis relative to the rolling element screw portion with which the rolling element nut is engaged, each of the rolling elements contacting a groove surface of a groove of the rolling element nut and a groove surface of a groove of the rolling element screw portion is rolled on the surfaces of the grooves at a constant speed in the same direction without contacting its adjacent rolling elements. Thus, it is possible to smoothly rotate the rolling element nut and the rolling element screw portion relative to each other with a low resistance.